A challenge is presented of replacing a portion of cis 1,4-polyisoprene rubber, particularly natural cis 1,4-polyisoprene rubber, with a synthetic polymer (other than synthetic cis 1,4-polyisoprene rubber) in a tire sidewall composition which is normally comprised of cis 1,4-polyisoprene rubber (particularly natural rubber) and cis 1,4-polybutadiene rubber to achieve a rubber composition of similar physical properties. A motivation for such challenge is a desire for a cis 1,4-polyisoprene rubber partial alternative, particularly a natural rubber partial alternative, in a form of a synthetic rubber (other than synthetic cis 1,4-polyisoprene rubber) to offset relative availability and/or cost considerations of cis 1,4-polyisoprene rubber, particularly natural rubber.
References made herein to synthetic rubber means synthetic rubber other than synthetic cis 1,4-polyisoprene rubber, unless otherwise indicated.
Therefore, such challenge has been undertaken to evaluate the feasibility of replacing a portion of cis 1,4-polyisoprene rubber, particularly natural rubber, in a tire sidewall with a synthetic rubber (other than a synthetic cis 1,4-polyisoprene rubber).
A simple substitution of a synthetic elastomer into a tire sidewall rubber composition which contains a significant cis 1,4-polyisoprene rubber, particularly natural rubber, content is not considered herein to be a feasible alternative where it is desired to achieve a rubber composition with similar physical properties.
It is to be appreciated that cis 1,4-polyisoprene rubber may be in a form of natural rubber and of synthetic cis 1,4-polyisoprene rubber (synthetic natural rubber). It is to be further appreciated that such natural rubber may be derived from, for example, trees and plants such as, for example, guayule plants and that such synthetic rubber may be derived from, for example, various chemical processes, whether by industrial, laboratory or other process.
While this presentation is primarily focused on replacement of a portion of natural cis 1,4-polyisoprene rubber in a rubber composition, it is intended herein that at least a portion of such natural rubber may be a synthetic cis 1,4-polyisoprene rubber.
In practice, pneumatic rubber tires conventionally have relatively thin rubber sidewalls which are normally expected to be able to be subject to significant punishment under typically operating conditions by undergoing considerable dynamic distortion and flexing, abrasion due to scuffing, fatigue cracking and weathering such as, for example, atmospheric ozone aging.
Significant physical properties for natural rubber/high cis 1,4-polybudadiene rubber based tire sidewall rubber compositions are considered herein to be rebound (at 100° C.) and tan delta (at 100° C.) which contribute to rolling resistance of the tire and therefore fuel economy of the associated vehicle, with higher values being desired for the Rebound property and lower values being desired for the tan delta property.
Additional desirable physical properties are considered herein to be higher low strain stiffness properties, in combination with the above rebound and tan delta properties, as indicated by Shore A hardness values and dynamic modulus G′ at 10 percent strain values at 100° C. to promote handling for the tire and resistance to sidewall wear.
Accordingly, it is readily seen that a partial substitution of a synthetic rubber for a portion of the natural rubber in such tire sidewall rubber composition is not a simple matter, and requires more than routine experimentation, where it is desired to substantially retain, or improve upon, a suitable balance of the representative physical properties of a natural rubber/high cis 1,4-polybutadiene sidewall rubber composition itself.
The aqueous emulsion polymerization prepared specialized styrene/butadiene rubber (E-SBR) for use in this invention for partial replacement of the natural rubber in the tire sidewall rubber composition contains bound styrene in a range of from about 12 to about 16 percent, and a Tg in a range of from about −60° C. to about −70° C. The specialized E-SBR has a Mooney (ML1+4), 100° C., viscosity in a range of from about 60 to about 100. A preferable specialized E-SBR has a bound styrene in a range of about 13 to about 15 percent in combination with a Mooney (ML1+4), 100° C., viscosity in a range from about 70 to about 90.
In the practice of this invention, the specialized E-SBR rubber has been observed to enable a partial replacement of the cis 1,4-polyisoprene rubber, particularly natural rubber, in a natural rubber-high cis 1,4-polybutadiene based tire sidewall compositions.
For this invention, a reference to glass transition temperature, or Tg, of an elastomer or sulfur vulcanizable polymer, particularly the specialized E-SBR polymer, represents an onset glass transition temperature of the respective elastomer or sulfur vulcanizable polymer in its uncured state. The Tg can be suitably determined by a differential scanning calorimeter (DSC) at a temperature rate of increase of 10° C. per minute, (ASTM 3418), a procedure well known to those having skill in such art.
A reference to Mooney (ML 1+4) viscosity of an elastomer or sulfur vulcanizable polymer, particularly the specialized E-SBR for use in this invention, represents the viscosity of the respective elastomer or sulfur vulcanizable polymer in its uncured state. The Mooney (ML 1+4) viscosity at 100° C. relates to its “Mooney Large” viscosity, taken at 100° C. using a one minute warm up time and a four minute period of viscosity measurement, a procedural method well known to those having skill in such art.
In the description of this invention, the terms “compounded” rubber compositions and “compounds”; where used refer to the respective rubber compositions which have been compounded with appropriate compounding ingredients such as, for example, carbon black, oil, stearic acid, zinc oxide, silica, wax, antidegradants, resin(s), sulfur and accelerator(s) and silica and silica coupler where appropriate. The terms “rubber” and “elastomer” may be used interchangeably. The amounts of materials are usually expressed in parts of material per 100 parts of rubber polymer by weight (phr).
Disclosure and Practice of the Invention
In accordance with this invention, a tire having a rubber sidewall is provided wherein said sidewall rubber composition is comprised of, based upon parts by weight per 100 parts by weight rubber (phr):
(A) about 10 to about 40, alternately about 15 to about 35 phr of natural cis 1,4-polyisoprene rubber (NR) having a Tg in a range of from about −60° C. to about −70° C., and
(B) about 5 to about 35 phr, alternately from about 5 to about 30, phr of a specialized emulsion polymerization prepared styrene/butadiene rubber (specialized E-SBR) having a Tg in a range of from about −60° C. to about −70° C. (similar to the Tg of NR) selected from:                (1) an E-SBR having a bound styrene content in a range of from about 12 to about 16 percent combined with a Mooney (ML1+4), 100° C., viscosity in a range of from about 60 to about 100, or, (more preferably),        (2) an E-SBR having a bound styrene content in a range of from about 13 to about 15 percent combined with a Mooney (ML1+4), 100° C., viscosity in a range of from about 70 to about 90;        
(C) (optionally) from zero to about 20, alternately about 5 to about 15, phr of synthetic cis 1,4-polyisoprene rubber;
(D) from about 50 to about 70 phr cis 1,4-polybutadiene rubber; and
(E) from about 30 to about 80, alternately from about 35 to about 70, phr of particulate reinforcing fillers comprised of:                (1) about 30 to about 80, alternately from about 35 to about 70, phr of rubber reinforcing carbon black, and, optionally;        (2) from zero to about 30, alternately from about 5 to about 20, phr of amorphous synthetic silica, preferably precipitated silica.        
The silica (e.g. precipitated silica), if used, may optionally, and if desired, be used in conjunction with a silica coupler to couple the silica to the elastomer(s), to thus enhance its effect as reinforcement for the elastomer composition. Use of silica couplers for such purpose are well known and typically have a moiety reactive with hydroxyl groups (e.g. silanol groups) on the silica and another moiety interactive with the elastomer(s) to create the silica-to-rubber coupling effect.
The E-SBR (specialized styrene/butadiene copolymer) may be prepared, for example, using conventional aqueous emulsion copolymerization of styrene and 1,3-butadiene monomers in an aqueous medium in the presence of an iron/hydroperoxide redox catalyst system (e.g. iron and hydroperoxide based) with a controlled styrene/butadiene monomer charge to achieve the desired bound styrene content of 12 to 16, or more preferably from 13 to 15 percent, in combination with the aforesaid Tg, (e.g. so long as the resulting styrene/butadiene copolymer rubber is said specialized E-SBR), which would not require undue experimentation by one having skill in such art.
As hereinbefore mentioned, a coupling agent may, if desired, be utilized with the silica to aid in its reinforcement of the rubber composition which contains the silica. Such coupling agent conventionally contains a moiety reactive with hydroxyl groups on the silica (e.g. precipitated silica) and another and different moiety interactive with the diene hydrocarbon based elastomer.
In practice, said coupling agent for said optional silica reinforcement, if used, may be, for example,
(A) a bis-(3-triakloxysilylalkyl) polysulfide such as, for example, a bis-(3-triethoxysilylpropyl) polysulfide, having an average of from 2 to about 4 and more preferably an average of from 2 to about 2.6 or from about 3.4 to about 4, connecting sulfur atoms in its polysulfidic bridge, or
(B) a bis-(3-triethoxysilylpropyl) polysulfide having an average of from about 2 to about 2.6 connecting sulfur atoms in its polysulfidic bridge or a bis-(3-triethoxysilylpropyl) polysulfide having an average of from about 3.4 to about 4 connecting sulfur atoms in its polysulfidic bridge, wherein said polysulfide having an average of from 2 to about 2.6 connecting sulfur atoms in its polysulfidic bridge (to the exclusion of such polysulfide having an average of from 3 to 4 connecting sulfur atoms in its polysulfidic bridge) is blended with said rubber composition in the absence of sulfur and sulfur vulcanization accelerator and wherein said polysulfide having an average of from about 3.4 to about 4 connecting sulfur atoms in its polysulfidic bridge is thereafter blended with said rubber composition in the presence of sulfur and at least one sulfur vulcanization accelerator, or
(C) an organoalkoxymercaptosilane composition of the general Formula (I) represented as:(X)n(R7O)3-n—Si—R8—SH   (I)
wherein X is a radical selected from a halogen, namely chlorine or bromine and preferably a chlorine radical, and from alkyl radicals having from one to 16, preferably from one through 4, carbon atoms, preferably selected from methyl, ethyl, propyl (e.g. n-propyl) and butyl (e.g. n-butyl) radicals; wherein R7 is an alkyl radical having from 1 through 18, alternately 1 through 4, carbon atoms preferably selected from methyl and ethyl radicals and more preferably an ethyl radical; wherein R8 is an alkylene radical having from one to 16, preferably from one through 4, carbon atoms, preferably a propylene radical; and n is an average value of from zero through 3, preferably zero, and wherein, in such cases where n is zero or 1, R7 may be the same or different for each (R7O) moiety in the composition, and
(D) said organalkoxyomercaptosilane of the general Formula (I) capped with a moiety which uncaps the organoalkoxymercaptosilane upon heating to an elevated temperature.
Representative examples of various organoalkoxymercaptosilanes are, for example, triethoxy mercaptopropyl silane, trimethoxy mercaptopropyl silane, methyl dimethoxy mercaptopropyl silane, methyl diethoxy mercaptopropyl silane, dimethyl methoxy mercaptopropyl silane, triethoxy mercaptoethyl silane, tripropoxy mercaptopropyl silane, ethoxy dimethoxy mercaptopropylsilane, ethoxy diisopropoxy mercaptopropylsilane, ethoxy didodecyloxy mercaptopropylsilane and ethoxy dihexadecyloxy mercaptopropylsilane.
Such organoalkoxymercaptosilanes may be capped with various moieties as discussed above.
A representative example of a capped organoalkoxymercaptosilane coupling agent useful for this invention is a liquid 3-octanoylthio-1-propyltriethoxysilane as NXT™ Silane from the GE Silicones Company.
The coupling agent may, for example, be added directly to the elastomer mixture or may be added as a composite of precipitated silica and such coupling agent formed by treating a precipitated silica therewith or by treating a colloidal silica therewith and precipitating the resulting composite.
For example, said optional silica (e.g. precipitated silica), or at least a portion of said optional silica, may be pre-treated prior to addition to said elastomer(s):
(A) with an alkylsilane of the general Formula (II), or
(B) with said bis(3-triethoxysilylpropyl) polysulfide having an average of from about 2 to about 4 connecting sulfur atoms in its polysulfidic bridge, or
(C) with said organomercaptosilane of the general Formula (I), or
(D) with a combination of said alkylsilane of general Formula (I) and said bis(3-triethoxysilylpropyl) polysulfide having an average of from about 2 to about 4 connecting sulfur atoms in its polysulfidic bridge, or
(E) with a combination of said alkylsilane of general Formula (II) and said organomercaptosilane of general Formula (I);
wherein said alkylsilane of the general Formula (J) is represented as:Xn—Si—R6(4-n)   (II)
wherein R6 is an alkyl radical having from 1 to 18 carbon atoms, preferably from 1 through 4 carbon atoms; n is a value of from 1 through 3; X is a radical selected from the group consisting of halogens, preferably chlorine, and alkoxy groups selected from methoxy and ethoxy groups, preferably an ethoxy group.
A significant consideration for said pre-treatment of said silica is to reduce, or eliminate, evolution of alcohol in situ within the rubber composition during the mixing of the silica with said elastomer such as may be caused, for example, by reaction such coupling agent contained within the elastomer composition with hydroxyl groups (e.g. silanol groups) contained on the surface of the silica.
Representative of additional synthetic diene based elastomers for said tire sidewall rubber composition are, for example, synthetic cis 1,4-polyisoprene rubber and styrene/butadiene copolymer rubber.
It is readily understood by those having skill in the art that the rubber compositions would be compounded by methods generally known in the rubber compounding art, such as mixing the various sulfur-vulcanizable constituent rubbers with various commonly used additive materials such as, for example, curing aids, such as sulfur, activators, retarders and accelerators, processing additives, such as oils, resins including tackifying resins, the aforesaid optional silica, and plasticizers, fillers, pigments, fatty acid, zinc oxide, microcrystalline waxes, antioxidants and antiozonants, peptizing agents and carbon black reinforcing filler. As known to those skilled in the art, depending on the intended use of the sulfur vulcanizable and sulfur-vulcanized material (rubbers), the additives mentioned above are selected and commonly used in conventional amounts.
The vulcanization is conducted in the presence of a sulfur-vulcanizing agent. Examples of suitable sulfur vulcanizing agents include elemental sulfur (free sulfur) or sulfur donating vulcanizing agents, for example, an amine disulfide, polymeric polysulfide or sulfur olefin adducts. Preferably, the sulfur-vulcanizing agent is elemental sulfur.
Accelerators are used to control the time and/or temperature required for vulcanization and to improve the properties of the vulcanizate. In one embodiment, a single accelerator system may be used, i.e., primary accelerator. In another embodiment, combinations of two or more accelerators in which the primary accelerator is generally used in the larger amount, and a secondary accelerator which is generally used in smaller amounts in order to activate and to improve the properties of the vulcanizate. Combinations of these accelerators have been known to produce a synergistic effect on the final properties and are somewhat better than those produced by use of either accelerator alone. In addition, delayed action accelerators may be used which are not affected by normal processing temperatures but produce satisfactory cures at ordinary vulcanization temperatures. Suitable types of accelerators that may be used in the present invention are amines, disulfides, guanidines, thioureas, thiazoles, thiurams, sulfenamides, dithiocarbamates and xanthates. Preferably, the primary accelerator is a sulfenamide. If a second accelerator is used, the secondary accelerator is preferably a guanidine, dithiocarbamate or thiuram compound.
The tire can be built, shaped, molded and cured by various methods which will be readily apparent to those having skill in such art.
The invention may be better understood by reference to the following example in which the parts and percentages are by weight unless otherwise indicated.